As a battery having a high energy density and excellent reliability, a nickel-metal hydride rechargeable battery, particularly, one which is provided with an unsintered positive electrode, in which an active material containing nickel hydroxide is filled into a foamed nickel porous body substrate or the like, is used in various uses. Further, in the unsintered positive electrode, since the electric conductivity of nickel hydroxide, which is an active material in a discharged state, is low, cobalt oxyhydroxide (CoOOH) having high electric conductivity is used as an electroconductive additive to increase the active material utilization. In this case, nickel hydroxide particles and cobalt compound particles are mixed for use, or the surface of the nickel hydroxide particle is coated with cobalt oxyhydroxide.
The nickel-metal hydride battery has a problem that the remaining capacity is easily reduced by self discharge. That is, a self-decomposition reaction of the nickel electrode is represented by the following chemical reaction formula:
thereby, the nickel electrode is reduced to nickel hydroxide involving oxygen-generation to cause self discharge.
Hence, various methods are proposed for inhibiting an oxygen-generation reaction to suppress self discharge by increasing an oxygen generation overvoltage.
In Patent Document 1, it is described that the oxygen generation overvoltage increases by containing a compound such as calcium with respect to a nickel electrode for an alkaline secondary battery, which has nickel hydroxide and a divalent or higher cobalt compound which coats the surface of nickel hydroxide, and contains a compound such as calcium in a coating layer thereof.
Further, an aqueous solution of potassium hydroxide which is superior in electric conductivity is often used for an electrolyte solution, but it is known that the oxygen generation overvoltage increases by replacing potassium with sodium having a smaller ion radius.
On the other hand, when the battery is overdischarged, a problem arises that the battery capacity is not completely recovered even if the battery is charged again. Cobalt oxyhydroxide is stable under a normal positive electrode condition and insoluble in an alkaline electrolyte solution. However, when a positive potential approaches a negative potential by overdischarge of a battery or a battery is in a state of reverse charge, cobalt oxyhydroxide is reduced, resulting in a decrease in an oxidation number of cobalt (Co) and a reduction in electric conductivity. If cobalt oxyhydroxide is further reduced to be changed to cobalt hydroxide, it is eluted into an electrolyte solution, and does not serve as an electroconductive additive.
For this situation, an attempt to inhibit the reduction of cobalt oxyhydroxide is made. In Patent Document 2 is proposed a constitution in which antimony or the like is added to an oxidized compound of cobalt. However, although a lot of substances to be added to an oxidized compound of cobalt are described in Patent Document 2, only a few substances of these substances, such as magnesium and aluminum, are actually measured and evaluated for the battery capacity as experimental data, and there is no description which allows to guess what characteristics other substances exhibit.